(1) Field of the Invention
The present invention relates to an optical communication device and an optical device which are used in optical communication, particularly to an RZ optical modulator which is of the optical communication device generating an RZ (Return to Zero) signal.
(2) Description of the Related Art
The optical modulators in which electro-optical crystals such as a LiNbO3 (lithium niobate (LN) ) crystal substrate and a LiTaO2 (lithium tantalate) crystal substrate are used can be cited as typical representatives of the optical communication device. For production of the optical communication device, an optical waveguide is formed by depositing a metal film on a part of the crystal substrate to perform thermal diffusion or by patterning the metal film to perform proton exchange in a benzoic acid solution, and then electrodes are provided near the optical waveguide.
For example, the optical waveguide includes an incident waveguide, a parallel waveguide and an outgoing waveguide. A signal electrode (also referred to as hot electrode) and a ground electrode are provided on the parallel waveguide, and a coplanar electrode is formed by the signal electrode and the ground electrode. In the case of the use of the substrate (Z-cut substrate) whose surface is cut out in a Z-axial direction of a crystal orientation, because a refractive-index change by an electric field in a Z-direction is utilized, the electrode is arranged immediately above the waveguide. At this point, the signal electrode and the ground electrode are patterned on each parallel waveguide, and a dielectric layer (buffer layer) is provided between the LN substrate and the signal electrode and ground electrode in order that a light propagating through the parallel waveguide is prevented from being absorbed by the signal electrode and the ground electrode. For example, SiO2 whose thickness ranges from 0.2 to 1 μm is used as the buffer layer.
When the optical communication device is driven at high speed, terminals of the signal electrode and ground electrode are connected with a resistor to form a progressive wave electrode, and a microwave electric signal is applied from the input side. At this point, phase difference between parallel waveguides (for example, A and B) is changed such that refractive indexes of two parallel waveguides A and B are changed by the electric field to +Δna and −Δnb respectively, which outputs the intensity-modulated signal light from the outgoing waveguide. An effective index of the microwave is controlled by changing the sectional shape of the electrode, and the speed of the light and the speed of the microwave are matched with each other, which allows light response characteristics to be obtained in a wide band.
As shown in FIG. 16, an example of the optical modulator which can generate the RZ (Return to Zero) signal includes the RZ optical modulator in which two optical modulators (intensity modulators) 100-1 and 100-2 are connected in tandem.
In the RZ optical modulator shown in FIG. 16, the first optical modulator 100-1 and the second optical modulator 100-2 are formed on a substrate 100 made of LN or LT. The first optical modulator 100-1 includes an incident waveguide 101, a first incident-side Y branching waveguide 102, first parallel waveguides 103A and 103B, a first outgoing-side Y branching waveguide 104, a first signal electrode 109 and a first ground electrode 110. A part of the first signal electrode 109 overlaps one (103A) of the first parallel waveguides 103A and 103B, and a part of the ground electrode 110 overlaps the other (103B) of the first parallel waveguides 103A and 103B. The second optical modulator 100-2 includes a second incident-side Y branching waveguide 105, second parallel waveguides 106A and 106B, a second outgoing-side Y branching waveguide 107, an outgoing waveguide 108, a second signal electrode 112, and a second ground electrode 113. A part of the second signal electrode 112 overlaps one (106A) of the second parallel waveguides 106A and 106B, and a part of the ground electrode 113 overlaps the other (106B) of the second parallel waveguides 106A and 106B. In FIG. 16 the numeral 111 designates the ground electrode 111.
In the RZ optical modulator having the above configuration, when a clock signal (microwave electric signal) 200 is supplied to the first signal electrode 109, the refractive index of the parallel waveguide 103A is changed in response to the voltage change in the clock signal, which generates the phase change in an incident light (CW light) propagating the parallel waveguide 103A. Therefore, in the outgoing-side Y branching optical waveguide 104, optical interference (constructive interference and destructive interference) occurs between the light from the parallel waveguides 103A and 103B to generate an optical clock signal (optical flashing signal) 300.
When a data (NRZ data) signal (microwave electric signal) 400 is supplied to the second signal electrode 112, similarly the refractive index of the parallel waveguide 106A is changed in response to the voltage change in the clock signal, which generates the phase change in the light (optical clock signal 300) propagating the parallel waveguide 106A. Therefore, in the outgoing-side Y branching optical waveguide 107, the optical interference occurs between the light from the parallel waveguides 106A and 106B to output an optical modulation signal (RZ signal) 500 from the waveguide 108. The optical modulation signal 500 has a waveform corresponding to a composite signal wave form of the clock signal 200 and the data signal 400.
In the optical modulator shown in FIG. 16, both the pre-stage and post-stage optical modulators 100-1 and 100-2 are formed as the intensity modulator. However, as shown in FIG. 17, sometimes the post-stage optical modulator 100-2 is formed as a phase modulator.
In the optical modulator shown in FIG. 17, the post-stage optical modulator 100-2 includes one waveguide (phase-modulation waveguide) 158, the signal electrode 112, and the ground electrodes 111 and 113. The waveguide 158 is coupled to the outgoing-side Y branching waveguide 104 of the pre-stage optical modulator 100-1 to form an interaction area. A part of the signal electrode 112 overlaps the waveguide 158. In FIG. 17, the constituent indicated by the same numeral represents the identical or similar constituent described above unless otherwise noted.
When the data (NRZ data) signal (microwave electric signal) 400 is supplied to the signal electrode 112 of the post-stage optical modulator 100-2, as with the optical modulator shown in FIG. 16, the phase modulation is performed to the clock signal from the pre-stage optical modulator 100-1 in response to the data signal 400, and the desired RZ signal can be obtained.
Further, the optical communication device having the waveguide structure includes the technologies proposed by Japanese Patent Application Laid-Open No. HEI 6-59291, Japanese Patent Application Laid-Open No. HEI 6-18735, and Japanese Patent Application Laid-Open No. 2001-109022.
Japanese Patent Application Laid-Open No. HEI 6-59291 discloses a waveguide type multiplexing/demultiplexing device. In order to prevent enlargement of the substrate, a connecting space is eliminated to realize miniaturization, and the number of production devices per one substrate is increased to achieve cost reduction. Therefore, the plural waveguide type multiplexing/demultiplexing devices are connected to one another using S-shaped curved waveguides and semi-circular waveguides so that the waveguide type multiplexing/demultiplexing devices are arranged adjacent to one another.
Japanese Patent Application Laid-Open No. HEI 6-18735 also discloses the waveguide type multiplexing/demultiplexing device. Input and output terminals of the plural Mach-Zehnder type multiplexing and demultiplexing device are arranged in one side to enable the miniaturization and the cost reduction. The Mach-Zehnder type multiplexing/demultiplexing device realizes multiplex transmission of four wavelength including two waves in a 1.3 μm band and two waves in a 1.5 μm band.
Japanese Patent Application Laid-Open No. 2001-109022 discloses an add-drop filter with switching function. The add-drop filter with switching function includes plural two-input and two-output type Mach-Zehnder interferometers. The two-input and two-output type Mach-Zehnder interferometer has two directional couplers or two 2×2 MMI (Multi Mode Interference) couplers, in which two optical waveguides formed on the substrate are brought close to each other. At least one of waveguides (arm portions) in the Mach-Zehnder interferometer includes an optically induced grating or a heater. The Mach-Zehnder interferometers in which the gratings are formed and thermo-optical switches are integrated on the silicon substrate, which realizes the miniaturization and low insertion loss.
In the RZ optical modulators shown in FIGS. 16 and 17, since the clock-signal optical modulator 100-1 and the data-signal optical modulator 100-2 are arranged in series in the light propagating direction, a chip length is doubled when compared with the NRZ optical modulator. As an interaction length, i.e. the lengths of the waveguides (arm portion) 103A and 103B (or 106A and 106B) is increased, drive voltage can be reduced. However, in the RZ optical modulator, because the interaction length is restricted by a chip size, there is a limitation in the reduction of the drive voltage.
Therefore, as shown in FIGS. 18 and 19, the clock-signal optical modulator 100-1 and the data-signal optical modulator 100-2 are arranged in parallel on the substrate 100, and the two optical modulator 100-1 and 100-2 (between the outgoing-side Y branching waveguide 104 and the incident-side Y branching waveguide 105 or a waveguide 158) are connected using a semi-circular folding (bending) waveguide 114. In FIGS. 18 and 19, the numeral 115 designates a groove portion 115 formed along an arc of the folding waveguide 114 in the substrate 100. Light entrapment in the folding waveguide 114 is enhanced by providing the groove portion 115, which allows the loss caused by leaky (radiated) light to be suppressed in the folding waveguide 114. In FIGS. 18 and 19, the constituent indicated by the same numeral represents the identical or similar constituent described above unless otherwise noted.
However, in the above configuration, a portion where the radiated light is generated is increased by the use of the folding waveguide 114 or by the increase in waveguide length, which generates a problem that the light insertion loss is increased. Since the technologies disclosed in Japanese Patent Application Laid-Open No. HEI 6-59291, Japanese Patent Application Laid-Open No. HEI 6-18735, and Japanese Patent Application Laid-Open No. 2001-109022 differ from the present invention in objects and application targets, even if these technologies exist, or even if theses technologies are collected, the problems cannot be solved.